1. Field of the Invention
The present invention relates to a printer, and more particularly, to an apparatus for and method of preventing paper double feeding in a paper feeding unit of a printer.
2. Description of the Related Art
In general, a printer or a copier includes a paper feeding unit which receives a plurality of sheets of paper and sequentially feeds the received sheets of paper.
FIG. 1 is a perspective view of a paper feeding unit of a printer employing a conventional apparatus for preventing paper double feeding, FIG. 2 is a side view schematically illustrating the paper feeding unit of the printer to explain an operation of the conventional apparatus shown in FIG. 1, and FIG. 3 is an enlarged view illustrating a front end of a sheet of paper fed in the apparatus of FIG. 2.
Referring to FIGS. 1 and 2, the paper feeding unit includes a paper cassette 20 which is installed to be detachably attached to a frame 10 of the printer, and on which a plurality of sheets of paper P are stacked, and a pickup roller 30 installed to be rotated with respect to the frame 10 of the printer and placed on an upper portion of the paper P stacked on the paper cassette 20. The pickup roller 30 is connected to a shaft 44 rotated by a driving motor 42 and to a gear group 46, is supported by a support arm 48 installed to pivot at the frame 10, and contacts the paper P stacked on the paper cassette 20 due to its weight. Also, an outer surface of the pickup roller 30 is formed of a material having a comparatively large friction coefficient so that the pickup roller 30 easily picks up the paper P. The paper feeding unit further includes a paper guide 50, which is fixed on the frame 10 of the printer and is placed at a front end of the paper cassette 20 to guide the paper P transferred into a printing unit of the printer by the pickup roller 30.
The apparatus for preventing the paper double feeding is provided in the paper feeding unit of the printer having the above structure. As shown in FIGS. 1-3, a stripper 52 generally formed of stainless plates or a synthetic material, such as TEFLON, as the apparatus for preventing the paper double feeding, is attached to a front side of the paper guide 50, that is, to a surface of the paper guide which the paper P contacts. The stripper 52 is installed to be inclined at a predetermined angle θ from a horizontal plane, to prevent double feeding of the paper P.
Referring to FIG. 3, if the pickup roller 30 that contacts a first sheet of paper P1 of the paper P stacked in the paper cassette 2 rotates, the first sheet of paper P1 is transferred into the printer by a friction force between the pickup roller 30 and the first sheet of paper P1. In this case, traveling of a second sheet of paper P2 is disturbed by the stripper 52 inclined at a predetermined tilt angle θ, so that only the first sheet of paper P1 is picked-up by the pickup roller 30 and can be transferred into the printer.
A paper feeding force Fp that acts on the first sheet of paper P1 due to the friction force between the pickup roller 30 and the first sheet of paper P1 may be obtained by Equation 1.Fp=μroll×N  (1)
Here, μroll represents a friction coefficient between the pickup roller 30 and the first sheet of paper P1, and N represents a vertical force that acts on the first paper P1 from the pickup roller 30.
Also, a second paper feeding force FD that acts on the second sheet of paper P2 due to another friction force between the first sheet of paper P1 and the second sheet of paper P2 may be obtained by Equation 2.FD=μpaper×N  (2)
Here, μpaper represents another friction coefficient between the sheets of paper P. Meanwhile, a paper feeding resistance force FN that disturbs feeding of the paper P is generated by the friction force between the paper P and a resistance generated by the stripper 52, and the paper feeding resistance force FN may be obtained by Equation 3.FN=μpaper×N+Fstripper  (3)
Here, Fstripper represents the resistance that acts on the paper P due to the stripper 52. The resistance Fstripper generated by the stripper 52 is proportional to the tilt angle θ of the stripper 52.
Likewise, the above-mentioned three forces are generated when the paper P is picked-up by the pickup roller 30 and is transferred into the printer. In order to prevent double feeding and non-picking up of the paper P, the three forces should satisfy the following conditions of Equation 4.Fp>FN>FD  (4)
More specifically, the paper feeding force Fp that acts on the first sheet of paper P1 should be larger than the paper feeding resistance FN, so that the first sheet of paper P1 is picked-up and is transferred into the printer. Accordingly, in order to satisfy the above conditions, the tilt angle θ of the stripper 52 should be properly set. If the tilt angle θ of the stripper 52 is too large, the paper feeding resistance force FN becomes larger than the paper feeding force Fp such that the first sheet of paper P1 is not picked-up.
Also, in order to prevent double feeding of the second sheet of paper P2 together with the first sheet of paper P1, the paper feeding resistance force FN should be larger than the paper feeding force FD that acts on the second sheet of paper P2. Thus, in order to satisfy the above conditions, the tilt angle θ of the stripper 52 is appropriately set.
That is, the stripper 52 as the conventional apparatus for preventing the paper double feeding has the tilt angle θ of about 70 degrees, large enough to satisfy the above-mentioned two conditions.
However, when an adhesive force is generated between the first sheet of paper P1 and the second sheet of paper P2 due to static electricity generated therebetween or an alien substance interposed therebetween, the paper feeding force FD that acts on the second sheet of paper P2 may be larger than the paper feeding resistance force FN. This is because since the paper feeding resistance force FN that acts on the second sheet of paper P2 is defined by the friction force between the second sheet of paper P2 and a third sheet of paper P3 and by the resistance Fstripper generated by the stripper 52, the paper feeding resistance force FN remains constant, whereas the adhesive force between the first sheet of paper P1 and the second sheet of paper P2 is added to the paper feeding force FD that acts on the second sheet of paper P2. In this case, a double feeding phenomenon occurs when the second sheet of paper P2 is transferred into the printer together with the first sheet of paper P1. The double feeding phenomenon may occur in a case of the third paper P3 or another paper thereunder. In this case, three or more sheets of paper are transferred into the printing unit of the printer.
As described above, the friction force or the adhesive force between the sheets of paper P is not always uniform. Thus, in order to prevent the paper double feeding, the tilt angle θ of the stripper 52 should be large. In this case, the paper feeding resistance force FN may be larger than the paper feeding force FP, so that the first sheet of paper P1 is not picked-up. Thus, the pickup roller 30 is rotated with a larger torque by using the driving motor 42 having a large capacity or controlling a deceleration ratio, so that the paper feeding force FP increases. In this case, a load on other elements of the paper feeding unit, such as the gear group 46, increases. Thus, these elements wear or break rapidly, and a life span of the elements is reduced.